Sergei Bezrukov writes:
This project does not directly involve a microcontroller. However, it is designed with a goal to power a PIC-based device. I used electronics from a garden light unit. This device has a solar cell mounted on the top if the light, an LED, a board with a circuit that turns LED on in the dark, and a battery. The battery block is a set of two AA Nickel Metal Hydride Batteries, the reverse engineered schematics of the control board is shown below. The photo cell has a high resistance in the dark (more that 1Mohm), while its light resistance is only about 150Ohm. This way the transistor, which serves as a key, is closed during the day time and the LED is not powered, allowing the solar cells to charge the battery.
Solar charger for three AAA batteries – [Link]